A significant cost factor in the manufacture of aircraft structures involves the preparation of holes to accept the wide variety of screws, rivets and other fasteners utilized in aircraft fabrication. It is not uncommon for over one million holes to be required in the manufacture of a large aircraft and this particular task represents a major portion of the production cost. In recent years improvements have been made in automating various processes in the aircraft manufacturing industry; however, the application of these automated techniques to the drilling of contour shaped subassemblies has sadly lagged.
A major problem involved in known attempts to automate the drilling and riveting equipment necessary for aircraft manufacture involves the fact that this drilling must be performed in assembly fixtures on the production floor. Typically, these drilling operations are currently performed manually because the equipment and controls necessary to realize an automated system which possesses the necessary complexity to operate on a contour structure have not yet been developed. Such a system must be capable of operating in five axes of motion in order to properly drill contoured skins of aircraft structure, since each hole must be drilled with the cutting tool normal to the contour at the point of entry. Further, such a system must be able to automatically adjust to ensure that the cutting implement is normal prior to penetration. Such a system would greatly reduce manufacturing costs on both metal and composite structures and result in a significant cost saving in the manufacture of large and small aircraft.
Recently, an automated drilling system has been disclosed in U.S. Pat. No. 3,973,859, issued to Huber et al. The Huber et al system teaches an automated technique for drilling compound curvature workpieces which are secured in a fixture by first scanning the workpiece with an optical camera to detect possible position variations. The optical camera is then removed and replaced with a tool manipulator head which includes a spherical bearing mounted between the cutting element and the nonworking end of the cutting tool. By manipulating the nonworking end of the cutting tool the cutting element can be rotated through an angular motion. However, since the rotational point for such manipulation lies at a point between the nonworking end of the cutting tool and the cutting element, the accuracy necessary for many aircraft operations is not available. Further, the complexity needed to calculate the position of the cutting element is increased due to the necessity of constantly calculating such position in view of the ratio of the distance between the manipulator and the spherical bearing and the distance between the spherical bearing and the surface of the workpiece.
A more reasonable approach to the problem of automated drilling operations is found in multi-axis numerically controlled or computer controlled drilling systems. However, prior art systems which incorporate computerized numerical control have generally been highly expensive, fragile and difficult to hold in a proper position. Therefore, it should be apparent, that a need has existed for a computerized numerically controlled drilling system which may be simply and accurately operated in five axes of motion and which may be easily held in a proper position once that position is obtained.